The right to request flexible working has been extended to all employees from today, where it was previously limited to just parents and carers.
The Department for Business, Innovation and Skills (BIS) said more than 20 million employees will benefit from the new legislation, which allows all employees who have been working for the same employer for 26 weeks or more the right to request.
Until today the right has only been available to parents with children under the age of 17 - or 18 if the child is disabled - and certain carers.
While the new rules have been largely welcomed by employment bodies and trade unions, some critics say they do not go far enough.
The Trades Union Congress (TUC) said the right to request will make it easier for all employees to better balance their work and home lives but it raised concerns that it was still too easy for employers to refuse any requests.
"Unfortunately the right to request is only the right to ask nicely. There is nothing to stop employers saying no. Of course not everyone in every company or organisation is able to work flexibly - some requests will always need to be turned down. But without the right to challenge employers, many workers will continue to lose out," said TUC general secretary Frances O'Grady.
Employers can refuse requests if there is a business reason for doing so. The reason must be included in a list provided by Acas which allows for eight grounds of refusal.
Acas also launched a Code of Practice which is designed to help employers consider any requests in a reasonable manner and a good practice guide including practical case studies.
BIS has predicted the new right will be particularly popular among older workers who may want to work differently as they approach retirement and to young people entering the labour market looking to take up additional training or learning while they work.
Business minister Jo Swinson said: "Firms that embrace flexible working are more likely to attract and retain the best talent and reap the benefits of a more motivated workforce. Employees will benefit from being able to balance work with other commitments in their lives. It also helps drive a cultural shift where flexible working becomes the norm."
Lawyers also warned of the potential legal risks that employers could face, explaining that there was a real risk of discrimination claims being brought forward which have been an issue in the past when dealing with flexible working requests.
"Going forward, the risk of this type of claim will continue unless an employer can demonstrate they've properly considered a flexible working request and turned down for verifiable good business reasons. It is possible also that different types of discrimination claims might be pursued," Charles Russell partner Bob Mecreate-Butcher explained.
"For example, a male employee whose request for flexible working has been turned down might argue they have been sexually discriminated against as their employer responds more sympathetically to requests from women with childcare responsibilities, rather than men. Age discrimination claims could also be pursued from those whose requests are turned down claiming their employer favours requests from those with childcare responsibilities, this having a disproportionate impact on those outside traditional child rearing age groups."
